Display devices, and in particular display screens, are currently undergoing many developments. Organic light emitting diodes, known as OLEDs, constitute a technology that has given rise to more luminous, less expensive and more effective display modules and will doubtless form the basis of the next generation of luminous screens. Nevertheless, this technology which should in principle be applicable in particular in the development of flat screens, is not yet sufficiently reliable and well understood. It should be recalled that a basic OLED cell consists of stacked, thin organic layers sandwiched between a transparent anode and a metal layer acting as a cathode.
Conventionally, the organic layers include a hole injecting layer, hole transporting layer, a photon-emitting layer and an electron transport layer. When a suitable current is applied to an OLED cell, the positive and negative charges recombine in the emitting layer to produce light. The structure of the organic layers and the type of anode and cathode used are chosen to maximise the recombination process in the emitting layer, which also improves light emission from the OLED device. OLED type display devices thus also lead to better presentation of information on lighter and less bulky screens. OLED technology is currently undergoing rapid developments but is still plagued by a number of difficulties.
One of the main approaches to developing OLED type diodes at present makes use of an indium oxide layer doped with tin (In2O3:Sn), generally known by the abbreviation ITO, as a transparent conducting layer acting as the anode. It is well known in the art that this ITO layer, deposited directly on the substrate and acting as an anode, has the advantages of excellent conduction and very good transparency, but unfortunately has the drawback, essentially because of the technology used to deposit it on glass substrates, of having a very rough surface which detracts from good functioning of the OLED device since this results in considerable heterogeneity in current density and even causes short circuits.
Another drawback of OLED type devices with an ITO layer is the tendency of indium atoms in the ITO layer to migrate under the effect of an electric field.
In addition, a drawback of the ITO layer is its low work function.
Numerous studies have been conducted to date in an attempt to resolve some of these drawbacks, at the same time as attempting to improve work function within the OLED cell without impairing anode conductivity too greatly.
In an attempt to reduce the roughness of the transparent layer deposited on the substrate, European application EP 0010786 therefore proposes subjecting the substrate to chemical treatment intended to diminish its roughness.
International application WO 0145182 suggests modifying the process for depositing the ITO layer in order to decrease surface roughness.
Different documents propose treating the ITO layer with plasma, as is the case, in particular, in international application WO 97/48115 and American patent U.S. Pat. No. 6,259,202.
International application WO 97/48115 proposes improving work function by treating the ITO layer with plasma.
American patent U.S. Pat. No. 6,262,441 describes using a semi-transparent metal layer in order to improve work function.
International application WO 01/15244 describes using an intermediate layer of a partially oxidized metal for substantially protecting the organic layers of the OLED cell from oxidation in the course of deposition of the ITO layer.
International application WO 99/13692 describes OLED type materials in which one or more polymer layers are inserted between the anode and the light emitting layers in order to increase work function. Similarly, patents U.S. Pat. No. 5,998,803, U.S. Pat. No. 5,714,838 and U.S. Pat. No. 6,087,730 describe OLED type devices containing an organic layer between the anode and the light emitting layers.
All the devices in the above-cited documents are aimed at reducing at least one of the known drawbacks of using ITO layers and, in particular at reducing the surface roughness of this layer and increasing the work function.
Consequently, all the above-cited documents attempt to improve the functioning of OLED devices by eliminating at least one of the above-mentioned drawbacks.
Among the devices cited in the literature, those that appear to be of particular interest contain a transparent ITO anode and organoluminescent layers, a conducting organic layer, in particular a layer having a doped polymer, such as poly(ethylenedioxy)thiophene, known as PEDOT, which improves the efficiency of the hole injection layer and reduces the unfavourable effect of roughness of the ITO layer.
Nevertheless, organic polymers, even if encapsulated, have the drawback of limited chemical stability and fairly low conductivity (typically about 1 to 5.10−2 Ω.cm versus 10−4 Ω.cm for ITO).